ConstraintLayout allows you to create large and complex layouts with a flat view
hierarchy (no nested view groups). It's similar to RelativeLayout in that all
views are laid out according to relationships between sibling views and the parent layout, but it's
more flexible than RelativeLayout and easier to use with Android Studio's Layout
Editor.

All the power of ConstraintLayout is available directly from the Layout Editor's
visual tools, because the layout API and the Layout Editor were specially built for each other.
So you can build your layout with ConstraintLayout entirely by drag-and-dropping
instead of editing the XML.

ConstraintLayout is available in an API library that's compatible with Android
2.3 (API level 9) and higher.
This page provides a guide to building a layout with ConstraintLayout in Android
Studio 3.0 or higher. If you'd like more information about the Layout Editor itself, see the
Android Studio guide to Build a UI with
Layout Editor.

Constraints overview

To define a view's position in ConstraintLayout, you must add at least one
horizontal and one vertical constraint for the view. Each constraint represents a connection or
alignment to another view, the parent layout, or an invisible guideline. Each
constraint defines the view's position along either the
vertical or horizontal axis; so each view must have a minimum of one constraint for each
axis, but often more are necessary.

When you drop a view into the Layout Editor, it stays where you leave it even if
it has no constraints. However, this is only to make editing easier; if a view has
no constraints when you run your layout on a device, it is drawn at
position [0,0] (the top-left corner).

In figure 1, the layout looks good in the
editor, but there's no vertical constraint on view C. When this
layout draws on a device, view C horizontally aligns with the left and
right edges of view A, but appears at the top of the screen because
it has no vertical constraint.

Figure 1. The editor shows view C below A, but it has no
vertical constraint

Figure 2. View C is now vertically
constrained below view A

Although a missing constraint won't cause a compilation error, the Layout Editor
indicates missing constraints as an error in the toolbar. To view the errors and
other warnings, click Show Warnings and Errors.
To help you avoid missing constraints, the Layout Editor can automatically add
constraints for you with the Autoconnect and infer
constraints features.

Add ConstraintLayout to your project

To use ConstraintLayout in your project, proceed as follows:

Ensure you have the maven.google.com repository declared in your module-level
build.gradle file:

In the toolbar or sync notification, click Sync Project with Gradle
Files.

Now you're ready to build your layout with ConstraintLayout.

Convert a layout

Figure 3. The menu to convert a layout to ConstraintLayout

To convert an existing layout to a constraint layout, follow these steps:

Open your layout in Android Studio and click the Design tab
at the bottom of the editor window.

In the Component Tree window, right-click the layout and
click Convert layout to ConstraintLayout.

Create a new layout

To start a new constraint layout file, follow these steps:

In the Project window, click the module folder and then select
File > New > XML > Layout XML.

Enter a name for the layout file and enter
"android.support.constraint.ConstraintLayout" for the Root Tag.

Click Finish.

Add a constraint

Start by dragging a view from the Palette window into the editor.
When you add a view in a ConstraintLayout, it displays a bounding box with square
resizing handles on each corner and circular constraint handles on each side.

Video 1. The left side of a view is constrained to the left side of the parent

Click the view to select it. Then click-and-hold one of the
constraint handles and drag the line to an available anchor point (the edge of
another view, the edge of the layout, or a guideline). When you release, the
constraint is made, with a default margin
separating the two views.

When creating constraints, remember the following rules:

Every view must have at least two constraints: one horizontal and one
vertical.

You can create constraints only between a constraint handle and an anchor
point that share the same plane. So a vertical plane (the left and right sides)
of a view can be constrained only to another vertical plane; and baselines can
constrain only to other baselines.

Each constraint handle can be used for just one constraint, but you can
create multiple constraints (from different views) to the same anchor
point.

Video 2. Adding a constraint that opposes an existing one

To remove a constraint, select the view and then click the constraint handle.
Or remove all the constraints by selecting the view and then clicking Delete Constraints.

If you add opposing constraints on a view, the constraint lines become squiggly
like a spring to indicate the opposing forces, as shown in video 2. The effect
is most visible when the view size is set to "fixed" or "wrap content," in which
case the view is centered between the constraints. If you instead
want the view to stretch its size to meet the constraints, switch the size to "match constraints"; or if you want
to keep the current size but move the view so that it is not centered, adjust the constraint bias.

You can use constraints to achieve different types of layout behavior, as described in
the following sections.

Parent position

Constrain the side of a view to the corresponding edge of the layout.

In figure 4, the left side of the view is connected to the left edge of the
parent layout. You can define the distance from the edge with margin.

Figure 4. A horizontal constraint to the parent

Order position

Define the order of appearance for two views, either vertically or horizontally.

In figure 5, B is constrained to always be to the right of A, and C is constrained below A.
However, these constraints do not imply alignment, so B can still move up and down.

Figure 5. A horizontal and vertical constraint

Alignment

Align the edge of a view to the same edge of another view.

In figure 6, the left side of B is aligned to the left side of A.
If you want to align the view centers, create a constraint on both sides.

You can offset the alignment by dragging the view
inward from the constraint. For example, figure 7 shows
B with a 24dp offset alignment.
The offset is defined by the constrained view's margin.

You can also select all the views you want to align, and then click Align
in the toolbar to select the alignment type.

Figure 6. A horizontal alignment constraint

Figure 7. An offset horizontal alignment constraint

Baseline alignment

Align the text baseline of a view to the text baseline of another view.

In figure 8, the first line of B is aligned with the text in A.

To create a baseline constraint, select the text view you want to constrain and then click
Edit Baseline, which appears below the view. Then click the text baseline and
drag the line to another baseline.

Figure 8. A baseline alignment constraint

Constrain to a guideline

You can add a vertical or horizontal guideline to which you can constrain views, and the
guideline will be invisible to app users.
You can position the guideline within the layout based on either dp
units or percent, relative to the layout's edge.

To create a guideline, click Guidelines
in the toolbar, and then click either Add Vertical Guideline
or Add Horizontal Guideline.

Drag the dotted line to reposition it and click the circle at the edge of the guideline to
toggle the measurement mode.

Figure 9. A view constrained to a guideline

Constrain to a barrier

Similar to a guideline, a barrier is an invisible line that you
can constrain views to. Except a barrier does not define its own position;
instead, the barrier position moves based on the position of views contained
within it. This is useful when you want to constrain a view to the
a set of views rather than to one specific view.

For example, figure 10 shows view C is constrained to the right side of a
barrier. The barrier is set to the "end" (or the right side in a left-to-right
layout) of both view A and view B. So the barrier moves
depending on whether the right side of view A or view B is is farthest right.

To create a barrier, follow these steps:

Click Guidelines in the toolbar, and then click
Add Vertical Barrier or
Add Horizontal Barrier.

In the Component Tree window, select the views you want inside the
barrier and drag them into the barrier component.

Select the barrier from the Component Tree, open the
Attributes window, and then set the barrierDirection.

Now you can create a constraint from another view to the barrier.

You can also constrain views that are inside the barrier to the
barrier. This way, you can ensure that all views in the barrier always align
to each other, even if you don't know which view will be the longest or tallest.

You can also include a guideline inside a barrier to ensure a "minimum"
position for the barrier.

Figure 9. View C is constrained to a
barrier, which moves based on the position/size of both view A and view B

Adjust the constraint bias

When you add a constraint to both sides of a view (and the view size for the same dimension is
either "fixed" or "wrap content"), the view becomes centered between the two constraints
with a bias of 50% by default. You can adjust the bias by dragging the bias
slider in the Attributes window or by dragging the view, as shown in video 5.

Adjust the view size

You can use the corner handles to resize a view, but this
hard codes the size so the view will not resize for different content or screen
sizes. To select a different sizing mode,
click a view and open the Attributes
window on the right side of the editor.

Near the top of the Attributes window is the view inspector, which includes controls for
several layout attributes, as shown in figure 10 (this is available only for views in a
constraint layout).

You can change the way the height and width are calculated by clicking the symbols indicated with
callout 3 in figure 10. These symbols represent the size mode as
follows (click the symbol to toggle between these settings):

Fixed: You specify a specific dimension in the text box below or by
resizing the view in the editor.

Wrap Content: The view expands only as much as needed to fit
its contents.

Match Constraints: The view expands as much as
possible to meet the constraints on each side (after accounting for the view's
margins). However, you can modify that behavior with the following attributes
and values (these attributes take effect only when you set the view width to
match constraints):

layout_constraintWidth_default

spread: Expands the view as much as possible to meet the
constraints on each side. This is the default behavior.

wrap: Expands the view only as much as needed to fit its
contents, but still allows the view to be smaller than that if
the constraints require it. So the difference between
this and using Wrap Content (above), is that setting the width to
Wrap Content forces the width to always exactly match the content
width; whereas using Match Constraints with
layout_constraintWidth_default set to wrap also
allows the view to be smaller than the content width.

layout_constraintWidth_min

This takes a dp dimension for the view's minimum width.

layout_constraintWidth_max

This takes a dp dimension for the view's maximum width.

However, if the given dimension
has only one constraint, then the view expands to fit its contents.
Using this mode on either the height or width also allows you to
set a size ratio.

Note: You cannot use match_parent
for any view in a ConstraintLayout. Instead use "match constraints"
(0dp).

Figure 11. The view is set to a 16:9 aspect with
the width based on a ratio of the height.

Set size as a ratio

You can set the view size to a ratio such as 16:9 if at least one of the view dimensions is set
to "match constraints" (0dp). To enable the ratio, click Toggle Aspect Ratio
Constraint (callout 1 in figure 10), and then enter the
width:height ratio in the input that appears.

If both the width and height are set to match constraints, you can click
Toggle Aspect Ratio Constraint to select which dimension is based on a ratio of the other.
The view inspector indicates which is set as a ratio by connecting the corresponding
edges with a solid line.

For example, if you set both sides to "match constraints", click
Toggle Aspect Ratio Constraint twice to set the width be a ratio of the height. Now the
entire size is dictated by the height of the view (which can be defined in any way)
as shown in figure 11.

Adjust the view margins

To ensure that all your views are evenly spaced, click Margin in the toolbar
to select the default margin for each view that you add to the layout.
Any change you make to the default margin applies only to the views
you add from then on.

You can control the margin for each view in the Attributes window by clicking
the number on the line that represents each constraint (in figure 10, callout 4 shows the bottom margin is set to 28dp).

Control linear groups with a chain

A chain is a group of views that are linked to each other with bi-directional position
constraints. For example, figure 13 shows two views that both have a constraint to each other, thus
creating a horizontal chain.

A chain allows you to distribute a group of views horizontally or vertically with the
following styles (as shown in figure 14):

Spread: The views are evenly distributed (after margins are accounted
for). This is the default.

Spread inside: The first and last view are affixed to the constraints on each
end of the chain and the rest are evenly distributed.

Weighted: When the chain is set to either spread or spread inside,
you can fill the remaining space by setting one or more views to "match constraints"
(0dp). By default, the space is evenly distributed between each view that's set to
"match constraints," but you can assign a weight of importance to each view using the
layout_constraintHorizontal_weight and layout_constraintVertical_weight
attributes. If you're familiar with layout_weight in a
linear layout, this works
the same way. So the view with the highest weight value gets the most amount of space; views
that have the same weight get the same amount of space.

Packed: The views are packed together (after margins are accounted for).
You can then adjust the whole chain's bias (left/right or up/down) by changing the chain's
head view bias.

The chain's "head" view (the left-most view in a horizontal chain and the
top-most view in a vertical chain) defines the chain's style in XML. However,
you can toggle between spread, spread inside, and packed by selecting any view
in the chain and then clicking the chain button that appears below the view.

To create a chain of views quickly, select them all, right-click one of the views,
and then select either Center Horizontally or Center Vertically, to create
either a horizontal or vertical chain, respectively (see video 4).

A few other things to consider when using chains:

A view can be a part of both a horizontal and a vertical chain, making it easy to build
flexible grid layouts.

A chain works properly only if each end of the chain is constrained to
another object on the same axis, as shown in figure 13.

Although the orientation of a chain is either vertical or horizontal, using one does not
align the views in that direction. So be sure you include other constraints to achieve the
proper position for each view in the chain, such as alignment
constraints.

Figure 13. A chain with just two views

Figure 14. Examples of each chain style

Video 4. Creating a chain from the action menu

Automatically create constraints

Instead of adding constraints to every view as you place them in the layout, you can
move each view into the positions you desire, and then click Infer Constraints to
automatically create constraints.

Infer Constraints scans the layout to determine the most
effective set of constraints for all views. It makes a best effort to constrain the views to
their current positions while allowing flexibility. You might need to make some adjustments to be
sure the layout responds as you intend for different screen sizes and orientations.

Autoconnect is a separate feature that is either on or off. When turned on, it
automatically creates two or more constraints for each view as you add them to the layout,
but only when appropriate to constrain the view to the parent layout. Autoconnect does not create
constraints to other views in the layout.

Autoconnect is disabled by default. You can enable it by clicking Turn on
Autoconnect in the Layout Editor toolbar.